Key takeaways
- Heat output = 500 × gallons per minute × temperature rise.
- Colder inlet water means a bigger rise, so a unit makes less flow in winter.
- A model's rating chart, not this formula, says what it will deliver.
Enter the hot water flow you want at the same time, and how many degrees the water needs to rise. The result is the heat output needed. It isn't a promise that any model will deliver it.
How the calculator works
BTU per hour = 500 × gallons per minute × temperature rise in °F
The 500 comes from about 8.33 pounds per gallon × 60 minutes per hour. To convert to electric power, divide by 3,412 for kilowatts. Temperature rise is your hot water setting minus your incoming water temperature.
This is standard water heating math, which we read on trade and retail pages rather than a government source.
Worked example
5.5 gallons per minute with a 70°F rise:
- 500 × 5.5 × 70 = 192,500 BTU per hour
- 192,500 ÷ 3,412 = about 56 kW if electric
That second number shows why electric whole-house units can run into a home's service limits. Ask an electrician before you plan on one.
Why winter matters
Flow and rise trade off. With the same output, a bigger rise means less flow. Using a unit able to add 150,000 BTU per hour, for example:
| Rise | Flow it could support |
|---|---|
| 40°F | 7.5 gpm |
| 60°F | 5.0 gpm |
| 80°F | 3.75 gpm |
So size for your coldest inlet temperature, and ask a local plumber what that is.
Not all flow is hot water
A shower mixes hot and cold, so it draws less than its full flow from the heater:
Hot share = (mixed temperature − cold) ÷ (hot − cold)
A 2.0 gpm shower at 105°F, with 120°F hot and 50°F inlet water, draws about 1.57 gpm of hot water, according to our math. Add up what you'd run at once using the hot share. We don't publish default fixture flows, as we haven't sourced them, so check your own fixtures.
Compare with a real model
- Work out your flow and coldest-weather rise.
- Find the model's published flow-at-rise chart.
- Check gas line, venting and electrical requirements with a licensed professional. Replacing a tank with a tankless unit can change all three, and may need a permit.
For a storage tank instead, use the first-hour rating worksheet.
Frequently asked questions
How big a tankless water heater do I need?
Work out the flow you'd use at once and your coldest temperature rise, then compare with the model's own chart.
How many showers can one run?
It depends on the unit, your shower flow and your inlet temperature. Use the math above with your own numbers.
Is the BTU rating the same as output?
Not always. Rated input and delivered heat can differ, so use the maker's flow chart for the final check.
Sources
We use primary sources where we can: building codes, federal rules and manufacturer documents. Read our editorial standards.
- MEP Academy: How to size tankless water heaters
- The Furnace Outlet: Tankless water heater sizing